Siemens 7SG164 Ohmega 400 Series Manual page 86

7SG164 Ohmega 400 Series Communications Interface
setting (see section 2.6) to allow use of easily available and relatively inexpensive commercial modems has
been provided. The downside to this is that the data security will be reduced slightly, and the system will not be
compatible with true IEC60870 control systems.
2.8.1 Connecting a modem to the relay(s)
RS232C defines devices as being either Data Terminal Equipment (DTE) e.g. Computers, or Data
Communications Equipment (DCE) e.g. Modems, where one is designed to be connected to the other. In this
case two DCE devices (the Modem and the Fibre-Optic Converter converter) together via a Null Terminal
connector which switches various control lines. The Fibre-Optic converter is then connected to the relay network
Tx to Relay Rx and Rx to Relay Tx.
2.8.2 Setting the Remote Modem
The exact settings of the modem are dependent on the type of modem preset. Although most support the basic
Hayes 'AT' command format, different manufacturers use different commands for the same functions. In
addition, some modems use DIP switches to set parameters, others are entirely software configured.
Before applying the following settings it is necessary to return the modem to its factory default settings, to ensure
it is in a known state.
There are several factors which must be set to allow remote dialling to the relays. The first is that the modem at
the remote end must be configured as Auto Answer. This will allow it to initiate communications with the relays.
Next the user should set the data configuration at the local port, i.e. baud rate and parity, so that communication
will be at the same rate and format as that set on the relay; and the error correction is disabled.
Auto-Answer usually requires 2 parameters to be set. The auto answer setting should be switched on and the
number of rings after which it will answer. The Data Terminal Ready (DTR) settings should be forced on. This
tells the modem that the device connected to it is ready to receive data.
The parameters of the modem's RS232C port need to be set to match those set on the relay, set baud rate and
parity to be the same as the settings on the relay, and number of data bits to be 8 and stop bits 1. Note, although
it may be possible to communicate with the modem at say 19200 BPS it may only be able to transmit over the
telephone lines at 14400. Therefore a baud rate setting that the modem can transmit should be chosen. In this
case, there is no14400 BPS on the relay choose the next lowest i.e. 9600 BPS.
Since the modems need to be transparent, simply passing on the data sent from the controller to the device and
vice versa, the error correction and buffering must be turned off.
In addition if possible force the Data Carrier Detect (DCD) setting to ON as this control line will be used by the
Fibre-Optic converter.
Finally these settings should be stored in the modem's memory for power on defaults.
2.8.3 Connecting to the remote modem
Once the remote modem is configured correctly, should it be possible to dial into it using standard configuration
from a local PC. As the settings on the remote modem are fixed, the local modem should negotiate with it on
connecting and choose suitable matching settings. If it does not, set the local modem to mimic the settings of the
remote modem described above.
©2011 Siemens Protection Devices Limited Chapter 6 Page 5 of 5